1. Field of the Invention
The present invention relates to ion-transport-enhancing materials that may be used as a liquid component/additive in conventional “liquid” batteries, in solid batteries, and/or gel battery electrolytes. More specifically, the present invention relates to additives that comprise cyclic phosphazenes with polyethers, polythioethers, or other groups that comprise ion-transport-enhancing groups. Furthermore, embodiments of the present invention relate to liquid components/additives that may be used to replace conventional electrolyte additives that have conventionally been volatile, unstable, flammable, and/or toxic, and therefore that have conventionally posed safety and health issues.
2. Background Art
A battery typically comprises one or more electrochemical cells connected in series, parallel or both, depending on desired output voltage and capacity. Batteries can be grouped in two categories. Primary batteries are single-use, non-rechargeable energy storage devices. Secondary batteries are rechargeable and are intended for multiple uses. However, all batteries have certain characteristics in common. They all have cathodes and anodes. There must also be an ion carrier that is capable of transporting ions between electrodes when the battery is charging or discharging.
The ion transport material can be an electrolyte solution or a polymer. In the lithium ion battery industry, both of these ion transport mechanisms are used in commercial batteries.
Cells featuring electrolyte solutions are typically called “Liquid” cells and are comprised of a cathode and anode separated by a porous polymer separator. An electrolyte solution placed between the electrodes transports ions between the cathode and the anode. The typical solvent is a mixture of organic carbonates such as ethylene carbonate, diethylene carbonate, etc, and the most common electrolyte is LiPF6. However, LiBF4 and LiClO4 can also be used.
Polymer cells have the same general design, except that the separator and the free solution are replaced by either a solid polymer electrolyte (SPE) or by a polymer “sponge” that is imbibed with traditional electrolyte solution. The advantages of polymer batteries are that they can be made to be thinner than conventional batteries. Also, because they do not have free solution, they do not require hard cases. Thus, they can be produced in flexible formats.
Battery cathodes are produced by applying a dispersion of finely-divided cathode powder and very finely divided carbon suspended in a solvent and polymer binder to a substrate. The most commonly used substrate is aluminum plated on a polyester film, such as available under the DuPont trademark MYLAR® (E. I. Du Pont De Nemours and Company WILMINGTON Del.). The resulting metalized film serves as a support for the cathode powder and as a current collector.
Anodes may be made of pure metal, alloys, or non-metallic powders coated on metalized film. In secondary lithium batteries, the film is copper-coated polyester. Non-metallic powders are applied to the film in a manner that is similar to that of the catodes.
A variety of cathodes and anodes can be used with lithium batteries. Primary batteries are most commonly comprised of manganese dioxide cathodes and lithium metal anodes. Secondary batteries typically use lithium cobalt oxide cathodes and carbon anodes. However, a large number of other cathode and anode materials have been described in the literature and some are commercially available. The following cathode materials are by no means an exhaustive list: lithium nickel oxide, lithium manganese oxide, lithium cobalt nickel oxide, and the like. Anodes could include lithium metal, carbon, oxides of titanium, or vanadium and the like.
In primary batteries, discharging occurs when lithium metal is oxidized to lithium cations, which are transported to the cathode where they move into the manganese dioxide crystal lattice. It is not safe to recharge these batteries. The reason is that repeated cycling with a lithium anode creates dendrites that grow out from the surface of the lithium metal. These whiskers will ultimately touch the cathode, creating a short circuit. For this reason, lithium metal is not a commonly-used anode for rechargeable lithium batteries.
The charge-discharge cycle of secondary batteries is similar to that of the primary battery. Lithium cations leave the anode and are transported to the cathode, typically LiCoO2, where they move into the crystal lattice. On charging, lithium cations are transported from the cathode to the anode. Carbon anodes are different from lithium metal anodes. Lithium is not actually plated on the carbon, as it is on lithium metal. Instead, the lithium ions are transported to specific sites in the carbon lattice where they form stable associations. A similar mechanism occurs with other non-metallic anodes.
By far the most common secondary lithium batteries are “liquid” cells. They are used in cell phone, computer, and camcorder applications. Of these, the cell phone applications are the most demanding. When cell phones transmit, a very large energy demand is placed on the battery. The electrochemical response is electricity production and release of lithium cations from the anode. Transport of lithium ions to the cathode becomes the rate limiting step in the process. Thus, excellent transport characteristics are necessary for the battery to function effectively. This is where liquid cells outperform polymer batteries. The solvent/electrolyte system that is employed in commercial lithium ion batteries provides very high lithium concentrations and low viscosity. Thus, transport is very good. Unfortunately, these systems also have some negative characteristics. Depending on the particular carbonate solvent(s) that is chosen, the flash point can range from 33° C. to 132° C. The other problem with them is that they are volatile. As the temperature rises, the vapor pressure also increases. These two characteristics can be very problematic in batteries that are subject to overheating.
Overheating can be a particular problem with lithium ion batteries depending on the application. When lithium ions intercalate into, or transport out of, the lithium cobalt oxide crystal, lattice shifts occur that release thermal energy. Normally this is not a problem. However, if a battery is under very heavy demand, the resultant heating can become significant. For example, if a cell phone with a lot of peripherals, such as a color screen or camera feature, is transmitting a large amount of data, the battery can undergo a significant amount of heating. As the battery heats up, the vapor pressure of the solvent increases. If the rate of thermal release is greater than the natural cooling of the battery, then the pressure could possibly exceed the structural limits of the case, leading to a rupture of the case. Under this scenario, the hot vapor may rapidly mix with oxygen in the air. Since the carbonate solvent in the battery would presumably be above their flash points, a fire could possibly result if an initiation such as a spark or other heat source were present.
Battery manufacturers have been aware of the potential for such an event since the beginning of the lithium ion battery industry. To counter the potential, they have included computerized fuses that create an open circuit when conditions in the battery are outside critical ranges. They also incorporate special membrane separators that lose their porosity when temperatures exceed critical amounts, thereby “shutting down” the battery.
Unfortunately, even with these safeguards, there have still been incidents of exploding cell phone batteries. Thus, it is clear that the safety measures employed by the battery industry might be improved upon.
Another battery system that could benefit from these new solvents according to embodiments of the invention is large lithium ion battery systems such as power tool batteries and hybrid electric vehicle batteries. Lithium ion batteries have been largely excluded from these markets due to the potential for explosion and fire. The use of phosphazene solvents according to embodiments of the invention could be enabling in these systems, eliminating the potential for battery rupture and fire.
The only way to substantively change the risk profile of lithium ion batteries is to change the components to materials that fundamentally diminish the probability of cell rupture and fire. A prime candidate for improvement is the solvent system, and the invented solvents address this need.